1. Field of the Invention
This invention relates to the field of infrared sensing, and more particularly to an integrated detector dewar cryoengine.
2. Description of Related Art
Infrared detectors are often used in conjunction with missiles and night vision systems to sense the presence of electromagnetic radiation having a wavelength of 1-15 .mu.m. Because they are often most sensitive when operating at approximately 77.degree. K., infrared detectors such as those fabricated from mercury-cadmium-telluride often require a cryoengine assembly to produce and maintain the required operating temperature. Such cryoengine assemblies are typically used in conjunction with an evacuated dewar in which the infrared detector is placed. The dewar is evacuated to remove thermally conductive gases which would otherwise occupy the region surrounding the detector. The dewar is cooled by placing an indented region ("coldwell") of the dewar in contact with an expansion chamber ("expander") of the cryoengine assembly which is supplied with a cooling fluid such as helium. The coldwell and the expander are separated by a gap to allow for variation in the dimensional characteristics of the dewar and expander, as well as to accommodate for differences in their thermal expansion characteristics.
To supply the expander with the cooling fluid, the cryoengine assembly often includes a compressor which delivers the fluid to the expander through a transfer line. As the fluid expands in the expander, it absorbs thermal energy from both the expander and the dewar causing the detector to cool. Because the temperature of the expander is related to the amount of fluid delivered by the compressor, the temperature of the detector may be controlled by varying the compressor speed.
While the infrared detector assemblies described above were able to produce and maintain the temperature levels required for sensitive operation of the mercury-cadmium-telluride detectors, they often had several disadvantages in terms of thermal conduction between the expander and the environment. Because the dewar and expander are structurally independent, the gap between the expander and the dewar acted as an insulator to prevent optimal conduction between the expander and the detector. In addition, the cross-sectional area through which cold loss occurred not only included the cross-sectional area of expander, but also included the walls of the dewar which were adjacent to the expander. These two features necessarily meant that the capacity of the cryoengine had to be somewhat larger than would otherwise be required.